1. Field of the Invention
The present invention relates to a retransmission control method and a retransmission control apparatus used in transmitting digital data sequences by radio by using orthogonal frequency division multiplexing (OFDM) signals.
2. Discussion of the Background
There is an increasing demand for indoor or outdoor radio data communication systems achieving high-speed data communications. In high-speed radio communication systems, radio signals disadvantageously travel to a receiver by more than one route because they are reflected by obstacles, such as buildings. This is referred to as xe2x80x9cmultipath interferencexe2x80x9d, resulting in significant degradation of receiving performance. It is therefore very important to decrease the multipath interference in the above type of system. A typical countermeasure being taken against this multipath interference is the use of an equalizer. However, this is not practical because the equalizer is too large for a high-speed radio communication system and seriously hampers the achievement of miniaturized and low-powered high-speed radio communication systems.
Thus, radio communication systems using the OFDM technique, which is robust to multipath interference, are being considered. In the OFDM technique, a multi-carrier transmission system is employed in which signals are transmitted while being superimposed on multiple carriers positioned at minimal orthogonal intervals. In this technique, influence from intersymbol interference caused by multipath distortion can be significantly suppressed, and thus, the quality of the receiving performance can be maintained. The use of the OFDM technique also significantly lowers the transmitting rate of each subcarrier and therefore makes it possible to modulate and demodulate multi-carrier signals by digital-signal batch processing (Fast Fourier Transformation (FFT) and Inverse Fast Fourier Transformation (IFFT)).
However, an OFDM signal waveform per unit time (OFDM symbol), which is also referred to as a time waveform, is like a noise. Thus, the amplitude fluctuation of a time waveform is greater in the multi-carrier transmitting method than in the single-carrier transmitting method. Accordingly, if the back-off amount of a transmission power amplifier is set to be small, nonlinear distortion by the power amplifier occurs, thus lowering the transmission quality, i.e., causing transmission errors.
On the other hand, if, considering a great level of amplitude fluctuation, the back-off amount of the transmission power amplifier is set to be large, transmission power cannot be amplified with high efficiency. A linearizer, which varies the back-off amount, may be used for making the power amplifier operable even with a small back-off amount. However, the use of a linearizer is not suitable because a linearizer is too large for a radio communication system, which hampers the achievement of a miniaturized, low-powered, and inexpensive radio communication system.
One of the solutions to the above problems may be to detect the peak power of each OFDM symbol so as to control the average power of OFDM symbols in accordance with the detected peak power. In this method, the time waveform of each OFDM symbol is normalized by the peak power so as to make the peak power of all the symbols uniform. This makes it possible to reduce the back-off amount of the transmission power amplifier.
According to this method, on the one hand, the transmission power amplifier can be operated with high efficiency, but on the other hand, the transmission quality varies according to the OFDM symbols. Therefore, the average power of OFDM symbols having a large amplitude fluctuation and a high level of amplitude is highly suppressed, thus degrading the transmission quality, i.e., encouraging transmission errors.
Therefore, in performing data communications between a radio base station and a radio terminal station, if transmission errors are detected, a request is made to retransmit the same data. However, since such transmission errors are caused by the time waveform of an OFDM symbol, errors may occur again to the retransmitted data, thereby causing a decrease in the throughput.
As discussed above, in the conventional OFDM transmitting method the transmission power amplifier can be operated with high efficiency without requiring a large-scale apparatus. In this method, however, the average power of OFDM symbols having a large amplitude fluctuation and a high level of amplitude is suppressed. As a consequence, the transmission quality is lowered, i.e., the transmission errors are encouraged. There is also a high possibility of causing errors in the data retransmitted in response to a retransmission request. This entails repeated transmission, thereby lowering the throughput.
Accordingly, in order to solve the above problems, it is an object of the present invention to provide a retransmission control method and a retransmission control apparatus in which the throughput is improved due to reduced retransmission errors by suppressing amplitude fluctuation of an OFDM time waveform and by improving the average transmission power when data is retransmitted.
In order to achieve the above object, according to one aspect of the present invention, there is provided a retransmission control apparatus for use in a radio communication system that uses an orthogonal frequency division multiplexing technique. The retransmission control apparatus includes transmitting means for transmitting to a radio station a data sequence, which serves as communication data that has been transformed into a time waveform of an orthogonal frequency division multiplexing symbol. Receiving means receives from the radio station a retransmission request for retransmitting the communication data. Transformation means transforms, in response to the retransmission request, the data sequence of the communication data to be retransmitted into an orthogonal frequency division multiplexing symbol having a time waveform different from the time waveform of the orthogonal division multiplexing symbol which had been sent before the retransmission request. Retransmitting means retransmits the communication data transformed by the transformation means to the radio station.
According to another aspect of the present invention, there is provided a retransmission control apparatus for use in a radio communication system that uses an orthogonal frequency division multiplexing technique. The retransmission control apparatus includes transmitting means for transmitting to a radio station a data sequence, which serves as communication data that has been transformed into a time waveform of an orthogonal frequency division multiplexing symbol. Receiving means receives from the radio station a retransmission request for retransmitting the communication data. Scramble means scrambles, in response to the retransmission request, the data sequence of the communication data to be retransmitted at least for the same communication data in accordance with a scramble sequence that varies every time the data sequence is retransmitted, and transforms the scrambled data sequence into an orthogonal frequency division multiplexing symbol having a time waveform different from the time waveform of the orthogonal division multiplexing symbol which had been sent before the retransmission request. Retransmitting means retransmits the communication data transformed by the scramble means to the radio station.
According to still another aspect of the present invention, there is provided a retransmission control apparatus for use in a radio communication system that uses an orthogonal frequency division multiplexing technique. The retransmission control apparatus includes transmitting means for transmitting to a radio station a data sequence, which serves as communication data that has been transformed into a time waveform of an orthogonal frequency division multiplexing symbol. Receiving means receives from the radio station a retransmission request for retransmitting the communication data. Reordering means reorders, in response to the retransmission request, the data sequence of the communication data to be retransmitted at least for the same communication data in accordance with a reordering method that varies every time the data sequence is retransmitted, and transforms the reordered data sequence into an orthogonal frequency division multiplexing symbol having a time waveform different from the time waveform of the orthogonal division multiplexing symbol which had been sent before the retransmission request. Retransmitting means retransmits the communication data transformed by the reordering means to the radio station.
According to a further aspect of the present invention, there is provided a retransmission control apparatus for use in a radio communication system that uses an orthogonal frequency division multiplexing technique. The retransmission control apparatus includes transmitting means for transmitting to a radio station a data sequence, which serves as communication data that has been transformed into a time waveform of an orthogonal frequency division multiplexing symbol. Receiving means receives from the radio station a retransmission request for retransmitting the communication data. Reordering means reorders, in response to the retransmission request, a spectrum signal obtained by modulating the data sequence of the communication data to be retransmitted at least for the same communication data, in accordance with a reordering method that varies every time the data sequence is retransmitted, and transforms the reordered spectrum signal into an orthogonal frequency division multiplexing symbol having a time waveform different from the time waveform of the orthogonal division multiplexing symbol which had been sent before the retransmission request. Retransmitting means retransmits the communication data transformed by the reordering means to the radio station.
According to a yet further aspect of the present invention, there is provided a retransmission control method for use in a communication system which is formed of a plurality of radio stations for transmitting and receiving communication data by using an orthogonal frequency division multiplexing technique. The retransmission control method which is employed by a transmitting radio station includes the steps of: receiving a retransmission request for retransmitting the communication data from a receiving radio station; and transforming, in response to the retransmission request, a data sequence to be retransmitted into an orthogonal frequency division multiplexing symbol having a time waveform different from the time waveform of the orthogonal frequency division multiplexing symbol which had been sent before the retransmission request, and transmitting the orthogonal frequency division multiplexing symbol as a retransmission signal in correspondence with the retransmission request.
According to a further aspect of the present invention, there is provided a retransmission control method for use in a communication system which is formed of a plurality of radio stations for transmitting and receiving communication data by using an orthogonal frequency division multiplexing technique. The retransmission control method which is employed by a transmitting radio station includes the steps of: receiving a retransmission request for retransmitting the communication data from a receiving radio station; and scrambling, in response to the retransmission request, a data sequence to be retransmitted for the same transmitting data in accordance with a scramble sequence that varies every time the data sequence is retransmitted, so as to obtain an orthogonal frequency division multiplexing symbol having a time waveform different from the time waveform of the orthogonal frequency division multiplexing symbol which had been sent before the retransmission request, and transmitting the orthogonal frequency division multiplexing symbol as a retransmission signal in correspondence with the retransmission request.
According to a further aspect of the present invention, there is provided a retransmission control method for use in a communication system which is formed of a plurality of radio stations for transmitting and receiving communication data by using an orthogonal frequency division multiplexing technique. The retransmission control method which is employed by a transmitting radio station includes the steps of: receiving a retransmission request for retransmitting the communication data from a receiving radio station; and reordering, in response to the retransmission request, a data sequence to be retransmitted at least for the same transmitting data in accordance with a reordering method that varies every time the data sequence is retransmitted, so as to obtain an orthogonal frequency division multiplexing symbol having a time waveform different from the time waveform of the orthogonal frequency division multiplexing symbol which had been sent before the retransmission request, and transmitting the orthogonal frequency division multiplexing symbol as a retransmission signal in correspondence with the transmission request.
According to a further aspect of the present invention, there is provided a retransmission control method for use in a communication system which is formed of a plurality of radio stations for transmitting and receiving communication data by using an orthogonal frequency division multiplexing technique. The retransmission control method which is employed by a transmitting radio station includes the steps of: receiving a retransmission request for retransmitting the communication data from a receiving radio station; and reordering, in response to the retransmission request, a spectrum signal obtained by modulating a data sequence to be retransmitted at least for the same transmitting data, in accordance with a reordering method that varies every time the data sequence is retransmitted, so as to obtain an orthogonal frequency division multiplexing symbol having a time waveform different from the time waveform of the orthogonal frequency division multiplexing symbol which had been sent before the retransmission request, and transmitting the orthogonal frequency division multiplexing symbol as a retransmission signal in correspondence with the retransmission request.